One of the most important things for optical quasar surveys is to efficiently
select the quasar candidates. In SDSS, quasar candidates are mostly selected based on
the multi-band optical photometric data [1]}. However, SDSS quasar
selection is very inefficient at redshifts between 2.2 and 3 due to the similar
optical colors of quasars with redshifts in this range to those of stars [2]}. One possible
way to improve this situation is to use the near-IR colors [2]}, [4]}, [5]}. Because quasars usually have a much flatter spectral energy
distribution over a wide range of wavelengths, their spectral shapes in the near-IR bands are
different from those of normal stars even if their optical spectra are similar to stars (e.g. for quasars
with redshifts between 2.2 and 3). [6]}
have demonstrated that by combining the UKIDSS near-IR colors with SDSS optical colors we can
separate well quasars from stars, and efficiently select quasars with redshift less than five. However, because UKIDSS/LAS will only cover
the sky area of 4000 \(deg^2\) , we have to think about other ways to improve the quasar
selection method. Here we investigate the cases of using the data in the WISE W1 and W2 bands, which
are close to the near-IR bands.
